Uniform impedance filter



p 1935. H. T. LYMAN, JR

I UNIFORM IMPEDANCE FILTER Filed Aug. 23, 1933 InVehCor:

Patented Apr. 16, 1935 UNITED STATES PATENT OFFICE UNIFonMfIMPsDANcn FILTER Harold T. Lyman, Jr., 'Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application August 23, mas/serial No. 686,421

, TClaims. My invention relates to filter circuits, particularly to filter circuits employed in connection with;

keyed or modulated-load circuitsof radio transmitters or the like, and its principal object is the provision of a filter system characterized by uni iorm impedance and adapted to ensure square- Wave keying in radio transmitters or similar ap-.

paratus but without the disadvantages heretofore encountered in the used such filter circuits for radio transmitter'or like uses.

In keyed radio transmitters or the like it is desirable, or practically necessary, that the filter circuits be such that the keying is absolutely square-wave, i. e., that the load current supplied from the generator be zero when the keying means or switch is open, and that the load current rise instantaneously to its final steady value as the keying means is closed and then drop instantlyto zero as soon as the keying means is reopened.

current to the load circuit has been relatively.

poor, or the efficiency of the system comprising generator, filter and load has been low.

In accordance with my present invention these difiiculties and others heretofore encountered in the use of the filter circuits employed in keyed radio transmitter systems or the like are obviated by the provision of special shunt filter elements associated with systems ofthis or similar .charact-er.

It will be understood, however, that my inven vided in accordance with my invention. Particularly these special filter elements in accordance with my invention maybe employed to great advantage in combination with special types of ripple suppressing filters for tuning out the main ripple from the supply current and forabsorbingv harmonics of the supply current ripple.

My invention will be better understood from the following description when considered in connection with the accompanying drawing and its scope will be pointed out in the appendedclaims.

Referring to the drawing;

Fig. l is a diagrammatic representation of an embodiment of my invention incorporating a current supply and filter system for a keyed radio transmitter, and having shunt elements in accordance with the invention.

Rig. 2 is a diagrammatic representation of an embodiment ofmy invention identical with that ofFig. 1 but including further special circuit ele-,

ments whereby the operation of the system of Fig.

1 is improved.

I Fig. 3 is a diagrammatic representation of an embodiment of my invention in which are combined ripple s ppressing circuits hereinabove mentioned and special shunt filter elements of the type illustrated in the embodiment of my invention shown in Fig. 2.

.Fig. 4 is a circuit diagram illustrating a further embodiment of my invention.

' It is known that in practically all conventional circuits composed of a generator of current of given" frequencies followed by a filter which suppresses all but a certain band of the frequencies produced by the generator and which is connected to a load, the impedance presented by the filter and generator to changes in the load circuit caused by changes in the load impedance or by other means such as variation of a voltage in series with the load, varies for different rates of change of the load current, the impedance, or the load modulation voltage. In other words, the filter and generator combined ime pedance varies with the frequency of the load modulation.

This variation of filter'and generator combined impedance makesit impossible for the load current to follow exactly the wave shape of the modulating voltage, assuming the modulating voltage to comprise a number of frequencies. In consequence two undesirable efiects arise, as follows: (1), Transient or complex wave distortion (sometimes known as linear distortion).

complex modulating voltage, such as that produced by opening andclosing a switch in series with the load, or by speech modulation of the load drawn by a transmitter, or by other means, will not: produce a load current exactly proportional at every instant to that modulating voltage. 7 Conversely, if some load characteristic be chosen so that a load current of given wave shape is drawn from the generator or source through the filter, the voltage required to produce the modulation will not follow the same wave shape as the load current. It is assumed that the load current is flowing through a pure resistance load, this latter assumption being justified in practically all cases under consideration for the reason that, if the load should be not resistive but reactive, the load current would not then follow the wave shape of the modulating voltage unless the reactance wave were tuned out at all frequencies by a special type of filter.

(2) Loss of load modulation.

Since the load is assumed to be purely resistive, any filter and current source impedance whatever will cause a loss of per cent load current variation for a given modulating voltage, whether this voltage is actually applied in series with the load, or whether this is the voltage applied by the modulating tubes in a transmitter modulator. This loss of per cent modulation will be a maximum at the frequency of modulation at which the filter and source combined impedance, presented to the equivalent modulation generator, is a maximum.

In accordance with my invention means are provided whereby the above undesirable effects are obviated and whereby the following objects are attained:

(1) increase of the percentage load current modulation for a given modulating voltage by reduction of the combined current source and filter impedance over the modulating frequency band.

(2) Improvement of transient and complex wave fidelity of load current with respect to the modulating voltage. In this connection it can be shown that the transient response of the standard inductance-capacity or LC filter for broadcast transmitters is relatively poor. In order to improve transient and complex wave fidelity it necessary to reduce the combined source and filter impedance to a value low with respect to the load resistance over the entire frequency band, i. e., from zero to infinity. Such reduction of the combined source and filter impedance automatically accomplishes also the above firstnamed purpose, 1. e., increase of the percentage load current modulation for a given modulation voltage.

(3) Reduction of the filter k. v. a. as compared to the total k. v. a. employed in usual LC filters. Means will be described hereinafter whereby complete suppression of the main or fundamental ripple or undesired frequency is accomplished by special tuned circuits, the remaining ripple harmonics being absorbed in the usual manner, and, in addition, current source and filter impedances are suppressed, all of the foregoing being accomplished with much less k. v. a. than that ordinarily employed in the present usual filters.

In Fig. l of the drawing, I have shown therein for purposes or" illustration, a radio telegraph transmitter of a conventional type, comprising an electron discharge amplifier I having a tuned grid circuit 2 and a tuned anode circuit 3, the latter being coupled to a suitable radiating system l. A source of high frequency oscillations 5 is coupled by means of transformer 6 to the input circuit of the amplifier I. The supply of oscillations from this source is arranged to be interrupted in accordance with telegraphic signals by means of key 1, or its equivalent. The amplifier E is provided with grid biasing potential from a voltage source indicated by the battery 3.

The amplifier I is further provided with anode potential from a current supply and filter means including shunt circuit elements in accordance with my invention. This current supply and filter system, shown for illustrative purposes in Fig. l and which is primarily for use in connection with mercury vapor rectifier or other low arc-drop rectifier devices, comprises a current source, indicated by the rectangle S and which is preferably a rectifier of the above-mentioned type having a smoothing condenser C1 in parallel therewith, connected through a filter means to the keyed or modulated-load circuit indicated generally by R1. and including amplifier I and the radiating system 4 to which the amplifier is coupled.

In a low arc-drop current supply system such as indicated in Fig. 1 it is necessary to accomplish the main part of the filtering by providing a reactor L1 between the rectifier or source S and the condenser C1. The resistance and reactance regulation of the current source may usually be neglected in analyzing the characteristics of the filter, since these regulation constants are ordi- L narily very small compared to the other constants. A resistor R1, which is equal to or greater than the reactance value of L1 at the main ripple frequency, is provided in parallel with L1. The presence of the resistor R1 is desirable for several reasons, one being that voltage surges across the rectifier tubes are reduced under conditions of abnormal or irregular tube emission, another reason being that the tuned impedance of L1 and C1 is reduced at the modulation tuning frequency. From the standpoint of transient fidelity, R1 tends to prevent low frequency oscillation when irregular load currents are drawn from the filter.

The filter circuit constituted by LiRl and Cl suppresses source ripple fairly well but is characterized by imperfect transient fidelity. To lower the initial rush of load current and to prevent the subsequent decay it is necessary to insert the reactor L2. The wave shape then is badly sloped and, in addition, the surge voltages on L2 are excessive under keyed load conditions. Resistor R2 is therefore added to correct these deficiencies. This resistor R2 permits an initial value of load current to be drawn instantaneously from the condenser C1. By proper choice of L2 and R2 it is possible to obtain the best wave shape, the proper relation being to make R2 equal to R1. (load resistance) and L2 equal to where C1 is in microfarads and L2 is in henries.

R2 reduces the voltage surges across L2 to negligible values, if the latter relation be employed.

It can be shown, however, that it is impossible to obtain uniform filter impedance with the filtering circuit so far described in connection with the system illustrated in Fig. 1; because of the non-uniformity of filter impedance the transient wave shape would still be relatively poor.

In accordance with my invention the difnculties above set forth in connection with the filter circuit so far described in connection with Fig. 1 are overcome by the provision of impedance suppressing shunts, illustrated in Figs. 1 and 2, these shunts being provided for the purpose and with the effect of causing the filter impedance to be low with respect to the load resistance over the entire frequency band, from zero to infinity.

Considering first the filter circuit system illusrated in Fig. 1, the impedance suppressing shunt provided therein in accordance with my invention is constituted by a circuit branch R3C2 having resistor R3 in series with condenser C2. This circuit branch RsCz is inserted for two reasons:

first, to remove the remaining slope in the wave impedance to an asymptotic value equal to RgRz V 3+R2 as the frequency increases. If, it were not for this shunt branch, the highfrequency impedance would approach asymptotically the value R2. Nu-

merically, C2 is equal approximatelyto Ci. More capacity than this gives no greater improvement in wave shape,while'a value less than thiscauses a sharp initial peak in the load current. Resistor R3 should be made equal to R2.

A value greater than this will cause a sloped'wave front, while a lower value will cause a sharp initial peak in the load current. Considering now the embodiment of my invention illustrated in Fig. 2, the. remaining low-frequency oscillation, and peak of low-frequency impedance, still present in the output of thesystem of Fig. 1 may be eliminated by the utilization, together with shunt branch RaCz, oia second type of shunt branch, shown in Fig. '2. Here R4L3C3,

having resistor R4, reactor and condenser C3 inseries, is a shunt broadly tunedto the frequency for which the filter shown in Fig. i has its peak impedance. This frequency will ,befvery close to the frequency,

. Numerically, this condition makes the alternating k. v. a. of L3 and C3 equal to that of L1 and C1. Reactor L3 is not, however, required to carry direct current, whichmakes its k. v.- a. rating, assuming 100% modulation of load current, approximately the rating of L1. The condenser voltage, being approximately modulated, re-

quires twice the k. v. a. rating of C1. 'The total k. v. a. of L303 is then 150% of the rating of LiC1.

ing the k. v. a. in the shunt RiLaCs. The loss of load modulation in the circuit system illustrated in Fig. 2 is very low at any frequency.

From the foregoing analysis of the systems shown in Figs. 1 and 2, it will be understood that diificulties'such as irregular filter impedance, low

circuit eiiiciency, loss of load modulation, and

transient or complex wave distortion, have been overcome by the provision of the uniform impedance filtercircuits comprising the shunt circuit elements described in connection with Figs. 1 and 2.

It will appear further, from the foregoing, that irregular filter impedance, loss of load modulation, and transient or complex wave distortion are inseparably linked, and thatto prevent both distortion and loss of load modulation the filter impedance must be reduced to a value low with respect to the load resistance, from zero to infinite frequency.

Reference will now be made more particularly to the employment of my invention in combination with special tuned circuits, hereinbefore mentioned, adapted to suppress the fundamental ripple, whereby large reduction of the filter k. v. a. compared to the total k. v. a. used in conventional LC filters may be effected.

Now it may be shown that the total additional k. v. a. required, in the system of Fig. 2, to reduce the filter impedance to a low value, in the manner above set forth in connection with the latter figure, is 178% referred to the total k. v. a; of L1Ci.. This is apparently a relatively large value. Even more than this additional percentage of k. v. a. is permissible, however, by the employment, in combination with the hereinbefore described shunt circuits shown in Figs. 1 and 2, of

apparatus which tunes out the main ripple and permits ordinary absorption filtering of harmonies of the main ripple, this apparatus replacing the conventional apparatus, such as the usual brute force filter, the purpose of which is to attempt to suppress, by absorption, all of the ripple frequencies including the fundamental.

Referring particularly to the embodiment of myinvention illustrated in Fig. 3, shunt circuit elements as described inconnection with Fig. 2 are shown in combination. with a circuit system capable of accomplishing the above-mentioned result, i. e.,.tuning out of the main ripple and permitting ordinary absorption filtering of the harmonics of the main. ripple. The system shown in Fig. 3 comprises a current source S such as a rectifier connected to and A. 0. supply line and providing a direct current upon which are superimposed ripple components including a .main ripple and harmonics thereof.

Current from the source S of Fig. 3 is supplied to a modulated-load circuit R1,, which may be similar to the load circuit R1. of Fig. 1, through a ripple-suppressing means including a transformer having a winding L4 in series between source S and load R1. and awinding L5 in series with a condenser C4, the latter winding and condenser being in parallel with the load RL. The ripplesuppressing and harmonic absorbing circuits further include a condenser C5 in parallel with the load circuit RL.

In the operation of the ripple-suppression and harmonic absorption system so far described'in connection with Fig. 3, the circuit L504 tunes to the fundamental ripple frequency and induces a back voltage in the circuit L4C5RL, this voltage being of the correct phase and magnitude, by proper reactor design and condenser adjustment, to suppress completely the flow of fundamental ripple current in the load R-L. At the same time the branch or circuit LiCt operates as a usual absorption filter for the harmonics ofthe main ripple frequency.

.The ripple-suppressing and harmonic-absorbing circuit above described including elements L4, L5, C4 and C5 is characterized, however, by the disadvantage that the current in the branch circuit L5C4 is affected by the load, which may vary and thereby upset the required conditions for zero load ripple. In order to overcome this disadvantage the reactive element L6 is added in series with transformer winding L4 and in inductive relation with windings Li and L5, the mutual inductance of L6, Li and L5 being designated by the character Miss.

It will be apparent that the addition of the reactive element L6 overcomes the tendency of the load current to upset the conditions producing zero fundamental ripple in the load. The reactance of Le, which is made equal to that of L4, is of sui'ficient value to ensure that the variation of the tuned impedance of L504, due to variation of the coupled-in load impedance does not raise appreciably the fundamental ripple voltage across LsCi, which is tuned slightly off resonance, on the capacitive side, to the fundamental frequency. These results in the system of Fig. 3 depend on fairly loose coupling of all of the inductive elements.

It can be shown that the ripple suppression and harmonic absor tion circuits of Fig. 3 effect a saving of approxhnately 70% of the filter k. v. a. of the con entional brute force filter.

The ripple-suppression and harmonic-absorption circuits hereinabove described in connection with 3, and circuits simflar thereto, will, however, be characterized by high impedance with respect to an equivalent modulation generator connected in series with the load. In order to make the source and filter impedance of ripple suppression and harmonic absorption systems such as above-described in connection with Fig. 3 low with respect to the load resistance over the entire frequency band, from zero to infinity, the systems which include these filter circuits are provided, in accordance with my invention, with impedance-suppressing shunts of the type hereinbefore described. In Fig. 3, for example, the ripple-suppression and harmonic-absorption filter circuits hereinabove described are shown as followed by impedance suppressing means including shunt circuit branch R302 of the type hereinbefore described in connection with Fig. 1 and a second shunt circuit branch R4L3C3 of the type described in connection with Fig. 2.

In order todetermine the net saving of k. v. a. effected by the combination of the ripple-suppression and harmonic-absorption circuits with the special shunt circuits, as illustrated in the system of Fig. 3, the k. v. a. ratings of the circuit elements required to suppress the modulation impedance of the filter system must be added. The net results in l:. v. a. saving are explained hereinbelow as follows:

As set forth hereinbefore in connection with the embodiment of my invention illustrated in Fig. 2 the additional k. v. a. required by the elements L2, R2, R3, C2, R4, L3, C3 of Fig. 2 is 178% of the la. v. a. of a standard LC filter tuning to a low frequency. Assuming first a system including a single phase full wave rectifier as current source, then if the filter incorporated in the latter system were tuned to the fundamental ripple, as in the filter included in the system of Fig. 3, the k. v. a. of this filter would be 14% of the rating of a conventional LC filter. The k. v. a. of the above-mentioned additional elements required for impedance suppression would then be 014x l78%:25% of that of the conventional LC filter.

The total 1:. v. a., therefore, of the combined filter system including a single phase full wave rectifier, as compared to a conventional filter K. v. a. of the ripple-sup- 5 pression filter circuit =30% k. v. a. of LC filter.

K. v. a. of the impedance suppressing elements =25% k. v. a. of LC filter.

Tota1 =55% of the k. v. a. of the conventional LC filter.

Assuming, second, a system including a three phase full wave rectifier, the k.v. a. required if the filter be tuned nearly to the fundamental ripple frequency is 8% of that of the conventional filter. The k. v. a. of the impedance suppressing 4 elements in this latter system is then 0.08 178%=14.3%, and the total k. v. a., for the same degree of ripple suppression, is 8%+14.3%= 22.3% of that of the system employing the conventional type of filter.

It may be shown, further, that in the case of certain other filters for systems including a three phase full wave rectifier, in which system a limit of 0.15% ripple has been set, the total k. v. a. for this degree of ripple suppression as well as 30 for the provision of low filter impedance over the whole frequency spectrum is, when circuit arrangements in accordance with my invention such as illustrated in Fig. 3 are employed, only 15% of that of the rectifier filter of the conventional type.

Referring now particularly to the modification of my invention illustrated in Fig. 4, shunt circuit elements in accordance with the invention are shown as incorporated in a system comprising a source of current, motor-generator MG, providing a direct current upon which ripple frequencies are superimposed, the inductance and resistance regulation constants of the generator being designated by L7 and R5. A modulated-load circuit R1,, which may be similar to that shown in Fig. l, is arranged to be supplied with current from the current source terminals TiTz. r

In order to suppress a given ripple frequency in the current supplied from the current source terminals TlTZ a two-element shunt filter circuit constituted by an inductance L8 and a condenser C6 in parallel is provided which is connected in series with a condenser C7, the parallel circuit L8C6 and the condenser C7 being connected in shunt with the load circuit R1,. To suppress the given ripple, the parallel element or circuit L806 is tuned nearly to the frequency of this ripple, on the inductive side of resonance, and then the whole shunt circuit constituted by Lace and condenser C7 is tuned to present nearly zero impedance across this shunt, to the given ripple frequency. 65

Now obviously the whole filter and generator circuit, so far described in connection with Fig. 4, presents zero impedance at zero and infinite frequencies to an equivalent modulation genera-tor. This impedance rises to the tuned parallel impedance of RsLvLaCsCv at some intermediate frequency very near to the ripple frequency. In order to reduce the tuned impedance to a low value, the elements La and C6 are modulated load circuit, the keying or' load modulating mechanism may comprisethe .modulation means including- ,switch I, as "shown for example in detail in Fig. 1. It will be readily understood, however, that the actual keying, or

I load modulating, may be accomplished not only directly as shown in Fig. l but, in general, by any suitable means.

What I claim as new and desire to secure by Letters Patent of the United States is: r

1. In combination, a source of direct current having a ripple component superimposed thereon, filter means to reduce said ripple component, a load circuit, means to modulate said load circuit over a band of frequencies, and a circuit in shunt with the load circuit and comprising a resistor and a condenser in series to reduce the impedance of said source and filter to a predetermined low'and uniform value, from zero to infinite frequency of the modulating frequency band. I

2. In combination, a source of direct current having ripple components superimposed thereon, filter means to reduce said ripple components,

a load'circuit, means to modulate said'load circuit over a band of frequencies, and means to reduce the impedance of said source and filter to a low value,-from zero to infinite frequency of said modulating frequency band, said last-named means comprising two circuits in shunt with the load circuit, one of said shunt circuits comprising a resistor and a condenser in series, the other of theshunt circuits being tuned to the frequency of the main ripple component of the direct current and comprising a resistor, a reactor and a condenser in series.

3. In combination, a source of direct current having superimposed thereon a main ripple component and harmonics of the main ripple, filter having ripple components superimposed thereon,

means tuned to the main ripple to suppress said main ripple and to absorb said harmonics, a load circuit, means to modulate said load circuit over a band of frequencies, and means to reduce the impedance of said source and filter to a value low with respect to the load resistance from zero to infinite frequency of said modulation band, said last-named means comprising a circuit having a resistor and condenser in series with'each other and in shunt with the load circuit and a second circuit tuned to the frequency of said main ripple having a resistor, a reactor and a condenser in series with each other and in shunt with the load circuit. r

'4. In combination, a source of direct current having superimposed thereon a main ripple component and harmonics of the main ripple, a load circuit, means to modulate said load circuit over a band of frequencies, filter means interposed betweenthe source and the load circuit comprising a transformer having onewinding in series between the source and the load circuit and a second winding in series with a condenser, said second winding and condenser being connected in shunt with the load circuit, said filter means furhe ac n e ,9? w fl s ame w ndi g nd onn ed in 5 here it second wi n a d s idco enseri bein tuned slightly on resonance onthe capacitive side to the c-fr uen o i i n a three md nss ns 9 1! c up an m an t reduce the impedance of source and said filter-means to; a value low with respect to the resistance from zero to infinite frequency ofssaid-m d o ban ai -i e -mme m s comprising two circuits in shunt to the load circuit, ,oneof said last-named shunt circuits in ch ding a resistor and a condenser in series, the other of said last-named circuits being tuned substantially to the frequency of said main ripple and including a resistor, a reactor and a condenser in series. 7

5. In combination, a source of direct current having ripple components superimposed thereon, a load circuit, means to modulate said load circuit over a band of frequencies, a filter circuit interposed between the source and the load circuit, said filter circuit comprising a condenser in shunt with the source and a reactor connected in series with the source and betweenthe source and said condenser, said reactor having a resistor in shunt therewith, the resistance value of said resistor being at least equal to the reactance value of said reactor at the main ripple frequency, said filter circuit further comprising a second reactor connected in series with the source and between said condenser and the load circuit, said second reactor having a resistor in shunt therewith, the resistance value of said second-named resistor being substantially equal to that of said load circuit, the reactance value of said second-named reactor being equal substantially to five divided by the reactance value of said condenser in microfarads, and a filter impedance suppressing circuit in shunt with said load circuit comprising a resistor and a condenser in series, the reactance of said last-named condenser in microfarads being substantially equal to one-fifth the reactance of said firstnamed condenser, the resistance value of said last-named resistor being substantially equal to two-thirds the resistance value of said secondnamed resistor.

6. In combination, a source of direct current a load circuit, means to modulate said load circuit over a band of frequencies, a filter circuit to c reduce said ripple components comprising a condenser in shunt with the source and two reactors in series therewith, one of said reactors being connected between the source and said condenser and the second reactor being connected between the condenser and the load circuit, a resistor in shunt with said one of the reactors and having a resistance value at least equal to the reactance value of said one of the reactors, a resistor in shunt with said second reactor and hav- ,thirds of that of said second-named resistor and a condenser having a reactance in'microfarads substantially equal to one-fifth the reactance of said first-named condenser, the second of said shunt circuits being broadly tuned to the main ripple frequency and comprising in series a resistor having a resistance value substantially that of said second-named resistor, a reactor, and a condenser.

7. In combination, a source of direct current having superimposed thereon a main ripple component and harmonics of the main ripple, a load circuit, means to modulate said load circuit over a band of frequencies, and means to suppress said main ripple and to absorb said harmonics and to reduce the impedance of said source and filter to a value low with respect to the load resistance from zero to infinite frequency of said modulation band, said last-named means comprising a circuit in shunt with the load circuit, said shunt circuit comprising a condenser in series with an inductance, a condenser and a resistance connected in parallel, a. parallel circuit element constituted by said inductance and said last-named condenser, said parallel circuit element being tuned first approximately to the frequency of said ripple and said parallel circuit element being tuned thereafter with said firstnamed condenser to cause said shunt circuit to present approximately zero impedance to said ripple frequency.

HAROLD T. LYMAN, Jr. 

